(1) Field of the Invention
This invention relates to an optical transmission system. The invention more particularly concerns a technique for generating local oscillator signals used in coherent communications systems, and other systems that both transmit and receive information at optical frequencies.
(2) Description of the Art
Conventional coherent optical communications systems, on reception of a received signal, downconvert the received signal to a frequency at which it is convenient to process it. This involves mixing the received signal with a local oscillator signal offset from the received signal by some convenient frequency. This will produce an output at the difference (downconverted) frequency, where typically lower cost components can be used for the subsequent processing. The advantage of this system over direct (incoherent) detection is that phase information is maintained in the downconverted signal, allowing synchronous detection methods to be used, and in addition can give the maximum level of performance in spectral regions where background noise or inefficient detector sensitivity lead to poor signal recovery.
One system where such optical processing is required is lidar. Here, an optical signal is transmitted to a target, and the reflections from the target are received and then processed to provide information relating to the target. Generally the transmitted signal is modulated in some way such that the returned signal contains additional information about the target, such as its range, velocity, vibration or reflectivity characteristics etc.
A common method of modulation used in lidar systems is to “chirp” the signal prior to transmission. A chirp is a frequency sweep of some form applied to the signal, where the sweep can be continuous or stepped discretely, or be a combination of both. A signal that contains a chirp can be processed in a manner that improves the range resolution of the system as compared to a non-chirped signal of equal pulse width. Traditionally, range resolution was improved in lidar systems by narrowing the transmitted pulse width. If the pulse is narrowed, then to maintain the same transmitted energy, and hence maintain the same range, the peak power of the pulse needs to be increased, which leads to costlier and larger systems. A chirped pulse of long duration can emulate the range resolution performance of a short pulse having the same energy, thus keeping the peak power low without sacrificing range performance. The bandwidth of the chirp is directly proportional to the improvement in range resolution of the system.
The technique is known as chirp pulse compression (CPC), as the returned signal is filtered in a manner that compresses the pulse. Before the filtering takes place the returned signal is downconverted to a lower frequency where signal processing is easier and cheaper. This is done by mixing the returned signal with a local oscillator (LO) that is offset in frequency from the returned signal by an appropriate amount.
Other modulation schemes include phase modulating the optical signal, or using some other form of modulation, such as amplitude modulation.
Generation of the local oscillator signal in such transmission systems has been implemented previously in two distinct ways. In one system a single laser has been used, the output of which is split into two. One part is then put through a modulator to add the modulation before being transmitted and reflected from a target, whilst the other is unmodulated and provides the LO signal. These two signals are then mixed together to produce the downconverted signal. This is described in more detail in Hulme, et al, Optical and Quantum Electronics, Vol 13, p35 (1981) An alternative approach is to use two lasers; one acts as the constant frequency local oscillator and the second is modulated and then transmitted as before. One example of this is the Firepond laser radar system, detailed in “Laser Radar” by A. Jellalian, Artech House, Boston (1992). The first technique has the disadvantage that the chirp bandwidth is limited to what can be achieved in an external (to the laser) modulator. External modulators are more limited in their modulation capabilities as compared to modulating a beam in the laser itself. The second method requires two separate lasers, and can suffer from instabilities due to frequency drift between them.
Optical signals are also used in communications systems, such as in telecommunications, and digital and analogue data transmission systems. These systems generally employ fibre optic transmission media, although some systems do transmit signals over free space. The detection of such signals is usually carried out in an incoherent manner, where no LO is required, which puts limitations on the minimum channel spacing.